Sucker-rod pumping is a long established method for artificially lifting crude petroleum and natural gas. In oil wells and in free-flowing gas wells, sucker rod pumping is used to lift liquids in the well, including crude oil in an oil well and water or other liquids that can fill up and block a free-flowing gas well. The components of a sucker-rod pumping system are immediately recognizable world-wide, especially the horse head and walking beam that commonly form the above-ground components of the subsurface pumping system. The above-ground components normally include a prime mover for providing driving power to the system, including gasoline and diesel engines and electric motors; a gear reducer for obtaining the necessary torque and pumping speed; a mechanical linkage for converting rotational motion to reciprocating motion, which includes the walking beam; a polished rod connecting the walking beam to the sucker-rod string; and a well-head assembly, sometimes referred to as a “Christmas tree,” which seals on the polished rod to keep fluids, including both natural gas and crude petroleum, within the well and includes a pumping tee for removing oil and gas to flow lines for storage and processing. Below ground, the downhole equipment may include a well hole casing; production tubing within the casing and through which the oil is withdrawn; a rod string centrally located within the downhole tubing and composed of sections of sucker rods coupled to provide the necessary mechanical link between the polished rod and the subsurface pump; a pump plunger typically comprising a traveling ball valve and connected directly to the rod string to lift the liquid in the tubing; and a pump barrel, which is the stationary cylinder of the subsurface pump and contains a standing ball valve for suction of liquid into the barrel during the upstroke. The downhole equipment may also include a sinker bar, which is a heavily weighted section of the rod string typically placed immediately above the pump plunger to drop the plunger quickly and with less vibration on the down stroke, thereby increasing pumping speed and efficiency.
Sucker rod sections typically vary in length depending on the well conditions and may be present in sections that are from shorter “pony rods” of 2 to 10 feet, particularly near the top of the well, to longer rods of 25 or 30 feet long or more, coupled to extend thousands of feet into the ground to reach an underground oil reservoir. The sucker rod sections typically have a long and slender central shaft portion with externally threaded “upset” ends, also called “pins,” of somewhat larger diameter to strengthen the joint. Sucker rods are joined end-to-end by much shorter, internally threaded, couplings or “rod boxes.” In the usual case, the coupling is of somewhat larger diameter than the long and slender shaft section of sucker rod in between couplings and may be of the same or larger diameter than the largest diameter of the sucker rod upsets. The larger diameter couplings are sometimes called “full sized couplings” and typically are designated by the initials “FS.”
Crude oil passes along the outside diameter of the sucker rod and around the couplings in the annular space between the sucker rod string and the inner surface of the production tubing in which the sucker rod string is contained. Natural gas typically flows in the annular space defined by the exterior surface of the production tubing and the inner surface of the well casing.
The sucker rod string, comprising couplings and rod segments, is surprisingly flexible. Tubing deviations from straight line are common and may include wells that have horizontal terminal segments. Lengthy sucker rod strings frequently abrade against the side of the production tubing and can wear the tubing and the sucker rod string and may result in breaking the rods and couplings. Pumping efficiency is reduced by frictional losses and down time for repairs and the repairs can be costly.
Numerous efforts have been made over the years to reduce abrasion, the impact of sucker rod and coupling wear, including the sucker rod and couplings wearing through the production tubing, and breaks in the rod string. For example, sucker rods and couplings may have centralizer guides with radially extending fins that contact the interior surface of the production tubing to space the rod from the tubing. The spaces between the fins provide flow channels for crude oil. Couplings may be specially shaped or covered to increase resistance to abrasion and wear.
It would be desirable to develop longer lasting, more efficient, tougher sucker rod strings that break less frequently, require less maintenance, and perform better in lifting crude oil and other fluids, including water. Water and other fluids can impede the free flow of natural gas, especially in the casing surrounding the production tube. It would also be desirable to develop such sucker rod strings that are readily and easily integrated into existing systems for pumping crude oil and into processes for the manufacture of sucker rods, couplings, and other components of the sucker rod string.